Optical Coherence Tomography (OCT) is an interferometric technique for performing high-resolution cross-sectional imaging that can provide images of samples including tissue structure on the micron scale in situ and in real time. OCDR is the one-dimensional analog of OCT. OCT is based on the principle of low coherence interferometry (LCI) and determines the scattering profile of a sample along the OCT beam by detecting the interference of light reflected or scattered from a sample and a reference beam. Each scattering profile in the depth direction (z) is called an axial scan, or A-scan. Cross-sectional images (B-scans), and by extension 3D volumes, are built up from many A-scans, with the OCT beam moved to a set of transverse (x and y) locations on the sample. OCT can be categorized into time-domain OCT (TD-OCT) and Fourier-domain OCT (FD-OCT). In time domain OCT (TD-OCT), the path length difference between light returning from the sample and reference light is translated longitudinally in time to recover the depth information in the sample. In frequency-domain or Fourier-domain OCT (FD-OCT), a method based on diffraction tomography, the broadband interference between reflected sample light and reference light is acquired in the frequency domain and a Fourier transform is used to recover the depth information. The sensitivity advantage of FD-OCT over TD-OCT is well established.
There are two common approaches to FD-OCT. One is spectral-domain OCT (SD-OCT) where the interfering light is spectrally dispersed prior to detection and the full depth information at a single transverse location on the sample can be recovered from a single exposure. The second is swept-source OCT (SS-OCT) where the source is swept over a range of optical frequencies and detected in time, therefore encoding the spectral information in time. In traditional point scanning or flying spot techniques, a single point of light is scanned across the sample. In parallel techniques, a series of spots (multi-beam), a line of light (line-field), or a two-dimensional field of light (partial-field and full-field) is directed to the sample. The resulting reflected light is combined with reference light and detected. Parallel techniques can be accomplished in TD-OCT, SD-OCT or SS-OCT configurations. OCT techniques have found great use in the field of ophthalmology.
In FD-OCT, the depth range over which an image is taken is inversely proportional to the spectral sampling interval of the spectral interferogram recorded by the system. Denser or finer sampling (more samples per wavenumber) enables longer depth range without aliasing as given by Nyquist sampling criterion (see for example, Lee et al., “Optimization for axial resolution, depth range, and sensitivity of spectral domain optical coherence tomography at 1.3 μm,” Journal of the Korean Physical Society, 2009). The spectral resolution (i.e., the smallest difference in wavelengths or wavenumbers that two spectral samples can be distinguished) is also an important factor in imaging over a certain depth range because it determines the depth-dependent sensitivity roll-off. If the spectral resolution is sufficiently high, then the signal beyond the depth range (i.e., signal over extended depth range) could be aliased into and collected in the depth range. Advances in light sources and detection systems have enabled higher spectral resolution and therefore imaging over extended depth range. Various embodiments of higher spectral resolution FD-OCT for extended depth range have been proposed and experimentally demonstrated in the past (see for example, U.S. Pat. Nos. 7,990,541; 9,163,930; EP 1,870,028; Jung et al., “Spectrally-sampled OCT for sensitivity improvement from limited optical power,” Optics Express, 2008; Tsai et al., “Frequency comb swept lasers,” Optics Express, 2009). While long depth range may be preferable in certain applications, for example wide-field retinal imaging, the high spectral resolution demands a large sampling number for full-depth high-axial-resolution imaging under the Nyquist sampling criterion. The large sampling number or data size can lower the acquisition and the processing speed, increase the system requirements, and present a memory burden.
Subsampling of FD-OCT data has been demonstrated as a compressive sensing method in extended depth imaging (see for example, U.S. Pat. No. 8,937,724; Siddiqui et al., “Optical-domain subsampling for data efficient depth ranging in Fourier-domain optical coherence tomography,” Optics Express, 2012). Subsampling is a well-known technique in telecommunication to down-convert the high-frequency signals to a lower-frequency baseband. In FD-OCT, subsampling aliases the high-frequency fringes to low-frequency ones and thus the image at the extended depth range is wrapped into the baseband depth window set by the sampling rate. In the prior works, quadrature detection was employed to detect the complex-valued interferogram which could avoid non-circular wrapping of extended depth signals, and the samples of limited depth extents were imaged without overlap in the baseband window. By concatenating copies of the baseband image, the actual image was assembled spanning an extended depth range, but many undesired duplicates of the image prevented a clear interpretation of the data. A surface finding procedure was suggested to recover the actual image, but the explicit depth ranging, which is localization of the true optical delay relative to the reference depth location, is undetermined from such a procedure. In addition, surface-finding procedures could work well for continuous structures such as the human retina, but will fail for non-continuous structures such as when imaging a surgical tool many millimeters above the surface of the tissue. Thus, there is a need for a method, a process, and/or an approach to efficiently collect and process the FD-OCT data for explicit ranging over extended depth.